Causality principle

This is a way of saying that time only goes in one direction. It states that a cause must always precede an effect. If an event (''the cause'') somehow influences another event (''the effect'') which occurs later in time, then the ''effect'' cannot in turn have an influence on the ''cause''. That is, event B must occur at a later time than event A. All frames must agree upon this ordering.

In simple terms, if we were to travel forward in time, see a murder, and then go back to the past and kill the killer, the future murder would be prevented. In other words, it has influenced its own cause, even though the event does not exist. This represents a causality violation, and it is the problem of dealing with such violations which leads most physicists to think that time travel and faster-than-light travel are impossible, at least in the general sense in which these terms are used in science fiction.

Against this, it may be remotely possible to build a Tipler machine, though it might take you a while to do so. The Tipler machine is an infinitely long cylinder, made of material as dense or denser than the matter in a neutron star. If this cylinder is rotated very rapidly, it will form closed timelike curves in the immediate area, which might allow time travel and possible subsequent violations of causality.

Written by Peter Macinnis

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